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"At the end of the day, the only person who cares about your trophy list is you".
That's a piece of advice I saw on another forum about a decade ago and it's as relevant now as it was then. As far as I'm concerned, if you can look over your trophy list and be happy with it then that's all that matters.
For myself, I've got a fair number of easy games on my profile that I imagine several people would scoff at. But at the end of the day, I had fun playing pretty much all of them and that's all that matters to me.

Celeste has a set of assist options that don't disable trophies.
It's a great game if you like 2D platformers and the assist options can be tweaked to either take the edge off of the difficulty or make the entire game a doddle that anyone could get the Platinum on.

You can definitely get the Platinum without online/co-op. The game - which is excellent BTW - can be pretty hard, but as you're a Souls veteran and you don't seem to mind grinding, you shouldn't run into too much trouble.
That said, having PS+ to get player revenants available to you is handy for farming good gear and it can be useful to help clear off a few of the DLC trophies quicker.
As for the missions that cycle every day, those are called Twilight Missions and they're basically just levels from the game with a different/harder enemy layout. They're pretty tough, but they're quite manageable solo if you keep your gear up to date and/or hold off on doing them until you hit the endgame.

Onryoki is a huge roadblock for new players because you simply don't have access to good gear or skills and Onryoki himself can both soak up damage and murder you with just a couple of hits. He's easy enough once you're familiar with the feel of the game, but until then he's a brick wall you just have to bash your head against.
If it's any consolation, I had huge problems with him at first as well. Unfortunately, there simply aren't any magic bullets to make things easier, Learning his patterns and using hit and run tactics is really the only option available to you this early in the game. I recommend hitting up youtube and watching how other players handled him. It could give you an idea of how - and how not - to handle him.

Putting points into whatever stats will boost your damage will help, but you'll need to grind out a lot of levels to make it noticable. On the plus side, a lot of grinding should net you plenty of purple gear to outfit yourself in which will definitely help.
Personally, my advice would be to boost your Heart stat to give you more Ki. That'll let you attack/dodge/block more before running out. Equip a Katana to take advantage of the Heart stat scaling it has to get decent damage. Then make sure your encumbrance is below 70% to give you a good dodge and then stay mobile and do hit and run tactics. Attack from behind when you can to bump up the damage as well. Once you learn which of his moves you can punish, he'll go down fairly easily.
Early game Nioh is all about learning attack patterns and punishing openings. Happily, the Onryoki is one of the hardest bosses in the game. If you can get him down, then you can get the Platinum.

Yeah, you can move three or four nodes per sphere level as long as you're retreading a path you've previously taken.
Also, teleport spheres are easy to get in the Fiend Arena and will let you hop back to nodes you already visited regardless of how far away they are.

The latest patch (v 1.02) dropped a couple of months ago. From what I've heard, it has indeed removed the rest mode trick.
I'm afraid that if you want the Platinum, you're going to have to do it the proper way.